The invention relates to solar modules having a transparent polyurethane front side and to a process for producing such modules.
By “solar modules” is meant photovoltaic components for the direct generation of electric power from sunlight. Key factors in cost-efficient generation of solar power are the efficiency of the solar cells used and the production cost and lifetime of the solar modules.
A solar module is conventionally composed of a front side made of glass, interconnected solar cells, an embedding material and a rear-side structure. The individual layers of the solar module have to fulfill the functions described more fully below.
The glass front side serves as protection against mechanical and atmospheric influences. It has to exhibit maximum transparency in order to minimize, as far as possible, absorption losses in the optical spectral region from 300 nm to 1150 nm, and hence efficiency losses of the silicon solar cells conventionally used for power generation. Hardened white glass (3 or 4 mm thick) with a low iron content having a transmittance in the aforesaid spectral region amounting to 90–92% is normally used.
The embedding material (EVA (ethyl-vinyl acetate) films are generally used) serves as the adhesive for the whole module composite. EVA melts during a lamination operation at about 150° C., flows into the gaps between the soldered solar cells and is thermally crosslinked. Formation of air bubbles which leads to reflection losses is prevented by lamination under vacuum.
The module rear-side protects the solar cells and the embedding material against humidity and oxygen. In addition, it serves as mechanical protection against scratching etc. during assembly of the solar modules and as electrical insulation. The rear-side structure may be made either of glass or, more commonly, of a composite film. In the main, the variants PVF (polyvinyl fluoride)-PET (polyethylene terephthalate)-PVF or PVF-aluminium-PVF are used.
The encapsulation materials used in solar module construction should, in particular, exhibit good barrier properties against water vapor and oxygen. Although the solar cells themselves are not attacked by water vapor or oxygen, corrosion of the metal contacts and chemical degradation of the EVA embedding material may take place. A broken solar cell contact leads to a complete failure of the module, since normally all of the solar cells in a module are interconnected in series electrically. A degradation of the EVA manifests itself in a yellowing of the module, combined with a corresponding reduction in power due to light absorption and a visual deterioration. Today about 80% of all modules are encapsulated on the rear side with one of the composite films described, and in about 15% of solar modules, glass is used for the front and rear sides. Where a composite film or glass is used for both the front and rear sides, highly transparent casting resins are sometimes used as embedding materials instead of EVA. These highly transparent casting resins cure slowly (several hours).
In order to achieve competitive production costs for solar power despite the relatively high capital costs, solar modules must achieve long operating times. Present-day solar modules are therefore designed for a service life of 20 to 30 years. In addition to high stability under atmospheric conditions, major requirements are made of the thermal endurance of the modules, whose temperature during operation may vary cyclically between 80° C. in full sunlight and temperatures below freezing. Solar modules are subjected to comprehensive stability tests (standard tests to IEC 1215) which include atmospheric tests (UV irradiation, damp heat, temperature change), hail tests and high voltage insulation tests.
Module construction, which accounts for 30% of the overall cost, represents a relatively high proportion of the production costs for photovoltaic modules. This large share in the module manufacture is caused by high material costs (hail-proof 3–4 mm thick front glass, multi-layer film on rear side) and by long process times, i.e. low productivity. In many cases, the individual layers of the module composite, that have been described above, are still assembled and aligned manually. In addition, the relatively slow melting of the EVA hot melt adhesive and the lamination of the module composite at approx. 150° C. under vacuum lead to production cycle times of 20–30 minutes per module.
Due to the relatively thick front glass pane (3–4 mm), conventional solar modules have a high weight, which makes stable and expensive holding structures necessary. The heat dissipation problem has also not been solved satisfactorily in present-day solar modules. Under full sunlight, the modules heat up to a temperature of 80° C., which leads to a temperature-induced deterioration of the solar cell efficiency and, in the final analysis, to an increase in the price of solar power.
Various attempts to reduce the module production costs by using cheaper (i.e. first and foremost, more rapid) production methods have not proven successful to date. In U.S. Pat. Nos. 4,830,038 and 5,008,062, the rapid foaming around the module rear side of thin-film solar modules with polyurethane foams by the RIM (Reaction Injection Moulding) method is described. Such thin-film solar cells are deposited (e.g. by chemical gas phase deposition) directly onto the rear side of the front glass of the solar module, thereby eliminating the need for an embedding material between the front- and rear-sides of the module. Currently, however, only about 10% of all solar cells are manufactured by thin film technology. The predominant solar cells are based on the technology of crystalline silicon wafers.